The immediate objective of this proposal is to describe the particle of tobacco mosaic virus (TMV) in molecular detail. This forms part of a broader objective: to study the process of macromolecular assembly; in particular, the assembly of macromolecular structures involving proteins and nucleic acids. Tobacco mosaic virus is the ideal system for such studies, since its biology and physical chemistry are already extensively studied, and it offeres the opportunity of relating its mechanism of assembly, and in particular the control of that assembly, to its molecular structure. X-ray diffraction provides an excellent method of obtaining an image of the virus in sufficient detail to describe the molecular structure and interactions. At present we have a partial model of the virus structure (all of the RNA but only 70% of the protein), and parts of this model are still highly tentative. Our goal is to improve the complete the model so that we can compare it in detail with structures of other aggregates, principally the "disk" (see below), in order to visualize the molecular details of switching between the different states of aggregation of the protein. The X-ray fiber diffraction pictures already available will be used to provide a data set of 3 A resolution , and the current model of Stubbs, Warren and Holmes, or an improved model, will be refined against that data set. Structures of a lead derivative of TMV and the U2 strain of TMV will also be refined, because of their importance in studying the carboxyl groups which form the control site in the assembly of TMV.